Variable temperature PXRD investigation of the phase changes during the dehydration of potassium Tutton salts

The thermal dehydration of the potassium Tutton salts K2M(SO4)2·6H2O (M = Mg, Co, Ni, Cu, Zn) was investigated using thermal gravimetric analysis (TG), differential scanning calorimetry (DSC), FTIR, and variable temperature powder X-ray diffraction. While each Tutton salts lost all six waters of hydration when heated to 500 K, the decomposition pathway depended on the divalent metal cation. K2Ni(SO4)2·6H2O lost all six waters in a single step, and K2Cu(SO4)2·6H2O consistently lost water in two steps in capped and uncapped cells. In contrast, multiple decomposition pathways were observed for the magnesium, cobalt, and zinc Tutton salts when capped and uncapped TG cells were used. K2Zn(SO4)2·6H2O lost the waters of hydration in a single step in an uncapped cell and in two steps in a capped cell. Both K2Mg(SO4)2·6H2O and K2Co(SO4)2·6H2O decomposed in a series of steps where the stability of the intermediates depended on the cell configuration. A greater number of phases were often observed in DSC and capped-cells TG experiments. A quasi-equilibrium model is presented that could explain this observation. These results highlight that experimental conditions play a critical role in the observed thermal decomposition pathway of Tutton salts.